1. Field of the Invention
The present invention relates to graphenes for use in electronic and optical device applications. The invention particularly relates to substrates having a graphene layer grown thereon, and electro-optical integrated circuits formed in such a substrate.
2. Description of Related Art
Graphenes (also called graphene sheets) are a sheet of six-membered rings which does not form a closed surface, and are formed by connecting numerous benzene rings two-dimensionally. Carbon nanotubes are formed by rolling up a graphene sheet into a tubular structure. Graphites are formed by stacking multiple graphene sheets. Each carbon atom in a graphene sheet has an sp2 hybrid orbital, and delocalized electrons are present at opposite surfaces of a graphene sheet.
The following typical physical properties of graphenes have been reported: (a) The carrier mobility is in the order of 200,000 cm2/Vs, which is one order of magnitude higher than those of silicon (Si) crystals and is also higher than those of metals and carbon nanotubes. (b) The 1/f noises of typical nanodevices can be significantly reduced. (c) The refractive index is negative. (d) The surface electrons behave as if they have no mass. Because of these properties, graphenes are identified as a candidate for post-silicon electronic materials.
In order to achieve graphene-based electronic and optical devices, a substrate having a graphene layer grown thereon is needed. Novoselov et al. reports a method for forming graphene on a substrate, in which a graphene film is separated from a highly oriented graphite crystal using an adhesive tape and then the removed graphene layer is transferred to the substrate. See e.g., K. S, NOVOSELOV, A. K. GEIM, S. V. MOROZOV, D. JIANG, M. I. KATSNELSON, I. V. GRIGORIEVA, S. V. DUBONOS, AND A. A. FIRSOV: “Two-dimensional gas of massless Dirac fermions in graphene”, Nature 438, 197 (2005).
Entani et al. reports a method for forming a graphite nanolayer on a platinum substrate by chemical vapor deposition using a specially designed ultrahigh vacuum apparatus. See. e.g., Shiro ENTANI, Susumu IKEDA, Manabu KIGUCHI, Koichiro SAIKI, Genki YOSHIKAWA, Ikuyo NAKAI, Hiroshi KONDOH, and Toshiaki OHTA: “Growth of nanographite on Pt(111) and its edge state”, Appl. Phys. Lett. 88, 153126 (2006).
Miyamoto et al. reports a method of growing, on an Si (110)Si substrate, an 80-nm thick preferentially (111)SiC oriented thin film of cubic silicon carbide (3C-SiC), and then thermally modifying the 3C-SiC thin film in ultrahigh vacuum. See. e.g., Yu MIYAMOTO, Maki SUEMITSU, Hiroyuki HANDA, and Atsushi KONNO: “Graphene/graphite formation by heat treatment of a 3C-SiC(111) thin film grown on a Si(110) substrate”, Conference Proceedings of the 69th Autumn Meeting at Chubu University, the Japan Society of Applied Physics, p. 404 (2008).
The above methods have the following problems: The method reported by Novoselov et al. is feasible on an experimental basis, but is not suitable for industrial applications because it is difficult for the method to provide large size substrates. The method reported by Entani et al. has an advantage in that the graphite nanolayer can be formed at relatively low temperatures (room temperature to 850 K). However, the method has a manufacturing cost problem because it requires a specially designed ultrahigh vacuum apparatus. The method reported by Miyamoto et al. has an advantage in which the graphene film can be formed on Si substrates. However, the method requires high temperature treatment (about 1350° C.) in ultrahigh vacuum, and therefore the type of substrate that can be used is limited and also the manufacturing cost is high.
In order to use a graphene film as a circuit conductor in electronic/photonic devices, the following techniques are at least required: a technique of growing a continuous graphene film having good electrical conduction; a technique of forming a circuit pattern in a graphene film; and a technique of preferentially growing a continuous graphene film only on a desired surface (e.g., on a desired circuit pattern).